1. Field of the Invention
This invention relates to a relay control circuit, or more in particular to a relay control circuit in which the welding of the contacts of the relay is prevented in a circuit where the on-off operation of the relay is controlled by the timer utilizing a commercial power supply as a reference clock pulse.
The present invention is suitably applied especially to the relay control circuit of a room air conditioner, a refrigerator and the like.
2. Description of the Prior Art
In conventional relay control circuits used with various devices, a timer circuit is controlled by use of a commercial power supply as a reference clock pulse, so that the relay is turned on and off by the output of the timer circuit. The phase of the output signal of the timer circuit for turning on and off the relay is synchronous with that of the reference clock pulse. As a result, the direction and phase of the current supplied from the commercial power supply which flows in the contact of the relay turned on and off are always identical, thus causing an accident of the welding of the relay contact.
A conventional relay control circuit shown in FIG. 1 will be described below with reference to the signal waveforms shown in FIGS. 2A to 2F.
In FIG. 1, reference numeral 1 shows a commercial power supply. At the output terminal of the commercial power supply 1, an AC voltage a as shown in FIG. 2B is generated. The voltage a, after being applied through a power supply circuit 2 including a transformer, is half-wave rectified by a half-wave rectifier circuit 3, which produces at the output thereof a signal b as shown in FIG. 2C. The half-wave rectified signal b is shaped in waveform by being sliced at the threshold level g at the waveform-shaping circuit 4, at the output of which is produced reference clock pulses c as shown in FIG. 2D. The reference clock pulses c are applied to the trigger terminal T of the frequency divider 6 of the timer circuit 15 including the frequency divider 6 and an RS flip-flop 5. In the timer circuit 15, numeral 7 shows a reset signal input terminal to which a reset signal f shown in FIG. 2A is applied. The reset signal f is generated by a push button which is adapted to be depressed when it is desired that current supply to the load or like be stopped.
A buffer amplifier 8 is turned on and off by the output of the timer circuit 15. When the amplifier 8 is turned on, the current flows in the coil 9 of the relay and the contact 10 of the relay is closed, so that the AC voltage e from the AC power supply 1 is applied to the load 11 and the current in phase with the voltage e flows through the contact 10.
Assume that current is flowing in the coil 9 with the output signal d of the timer circuit 15 kept at high level H and that the reset signal f is supplied to the reset signal input terminal 7 without regard to the reference clock pulses c. The frequency divider 6 and the RS flip-flop 5 are reset by the reset signal f, with the result that the output signals of the frequency-divider 6 and the RS flip-flop 5 are reduced to L level. In other words, the output signal d of the timer circuit 15 is reduced to level L the instant the reset signal f is applied thereto as shown in FIG. 2A. After the lapse of time T.sub.1 of the timer operation stoppage which is set by the number of stages of the frequency divider 6, a pulse produced from the Q output terminal of the frequency divider 6 sets the RS flip-flop 5, thus raising the timer output signal d to H level. The timer output signal d is raised to H level in the same timing as the output signal of the frequency divider 6 is raised to H level. The timing at which the output signal of the frequency divider 6 is raised to H level, in turn, is identical to that of rise of the reference clock pulse c. In other words, the phase in which the output signal d of the timer circuit 15 is reduced to L level is determined by the timing of the reset signal F at random, but the output signal d of the timer circuit 15 is raised to H level at the same timing as the rise of the reference clock pulse c.
Assume that it takes time T.sub.2 before the contact 10 is closed by the current flow in the coil 9 with the turning on of the buffer amplifier 8 after the output signal of the timer circuit 15 is raised to H level. The contact 9 is closed after the lapse of time T.sub.2 following the time point when the output signal of the timer circuit 15 is raised to H level, i.e., after the lapse of time T.sub.2 following the time point when the reference clock pulse c is raised to H level. The phase of the AC voltage a associated with the closing of the contact is always identical as shown by the arrow. Accordingly, the voltage e supplied to the contact 10 of the relay, namely, the current flowing in the contact 10 is always in the same direction and same phase as shown in FIG. 2F. That is, it is identical to the supply of the DC current, with the result that transfer of the contact occurs. This shortens the service life of the contact and finally causes the welding thereof.